Carcinoma of the cervix is the second most common neoplasm in women, accounting for approximately 12% of all female cancers and causing approximately 250,000 deaths per year. Baldwin et al. (2003) Nature Reviews Cancer 3:1–10. In many developing countries where mass screening programs are not available, the clinical problem is more serious. Cervical cancer in these countries is the number one cause of cancer deaths in women.
The majority of cases of cervical cancer represent squamous cell carcinoma, although adenocarcinoma is also seen. Cervical cancer can be prevented by population screening as it evolves through well-defined noninvasive intraepithelial stages, which can be distinguished morphologically. Williams et al. (1998) Proc. Natl. Acad. Sci. USA 95:14932–14937. While it is not understood how normal cells become transformed, the concept of a continuous spectrum of histopathological change from normal, stratified epithelium through cervical intraepithelial neoplasia (CIN) to invasive cancer has been widely accepted for years. The precursor to cervical cancer is dysplasia, also known in the art as CIN or squamous intraepithelial lesions (SIL). Squamous intraepithelial abnormalities may be classified by using the three-tiered (CIN) or two-tiered (Bethesda) system. Under the Bethesda system, low-grade squamous intraepithelial lesions (LSIL), corresponding to CINI and HPV infection, generally represent productive HPV infections with a relatively low risk of progression to invasive disease. High-grade squamous intraepithelial lesions (HSIL), corresponding to CINII and CINIII in the three-tiered system, show a higher risk of progression to cervical cancer than do LSIL, although both LSIL and HSIL are viewed as potential precursors of malignancy. Patient samples may also be classified as ASCUS (atypical squamous cells of unknown significance) or AGUS (atypical glandular cells of unknown significance) under this system.
A strong association of cervical cancer and infection by high-risk types of human papilloma virus (HPV), such as types 16, 18, and 31, has been established. In fact, a large body of epidemiological and molecular biological evidence has established HPV infection as a causative factor in cervical cancer. Moreover, HPV is found in 85% or more of the cases of high-grade cervical disease. However, HPV infection is very common, possibly occurring in 5–15% of women over the age of 30, but few HPV-positive women will ever develop high-grade cervical disease or cancer. The presence of HPV alone is indicative only of infection, not of high-grade cervical disease, and, therefore, testing for HPV infection alone results in many false positives. See, for example, Wright et al. (2004) Obstet. Gynecol. 103:304–309.
Current literature suggests that HPV infects the basal stem cells within the underlying tissue of the uterine-cervix. Differentiation of the stem cells into mature keratinocytes, with resulting migration of the cells to the stratified cervical epithelium, is associated with HPV viral replication and re-infection of cells. During this viral replication process, a number of cellular changes occur that include cell-cycle deregulation, active proliferation, DNA replication, transcriptional activation and genomic instability (Crum (2000) Modern Pathology 13:243–251; Middleton et al. (2003) J. Virol. 77:10186–10201; Pett et al. (2004) Cancer Res. 64:1359–1368).
Most HPV infections are transient in nature, with the viral infection resolving itself within a 12-month period. For those individuals who develop persistent infections with one or more oncogenic subtypes of HPV, there is a risk for the development of neoplasia in comparison to patients without an HPV infection. Given the importance of HPV in the development of cervical neoplasia, the clinical detection of HPV has become an important diagnostic tool in the identification of patients at risk for cervical neoplasia development. The clinical utility of HPV-based screening for cervical disease is in its negative predictive value. An HPV negative result in combination with a history of normal Pap smears is an excellent indicator of a disease-free condition and a low risk of cervical neoplasia development during the subsequent 1–3 years. However, a positive HPV result is not diagnostic of cervical disease; rather it is an indication of infection. Although the majority of HPV infections is transient and will spontaneously clear within a 12-month period, a persistent infection with a high-risk HPV viral subtype indicates a higher risk for the development of cervical neoplasia. To supplement HPV testing, the identification of molecular markers associated with cervical neoplasia is expected to improve the clinical specificity for cervical disease diagnosis.
Cytological examination of Papanicolaou-stained cervical smears (Pap smears) currently is the method of choice for detecting cervical cancer. The Pap test is a subjective method that has remained substantially unchanged for 60 years. There are several concerns, however, regarding its performance. The reported sensitivity of a single Pap test (the proportion of disease positives that are test-positive) is low and shows wide variation (30–87%). The specificity of a single Pap test (the proportion of disease negatives that are test-negative) might be as low as 86% in a screening population and considerably lower in the ASCUS PLUS population for the determination of underlying high-grade disease. See, Baldwin et al., supra. A significant percentage of Pap smears characterized as LSIL or CINI are actually positive for high-grade lesions. Furthermore, up to 10% of Pap smears are classified as ASCUS (atypical squamous cells of undetermined significance), i.e., it is not possible to make a clear categorization as normal, moderate or severe lesion, or tumor. However, experience shows that up to 10% of this ASCUS population has high-grade lesions, which are consequently overlooked. See, for example, Manos et al. (1999) JAMA 281:1605–1610.
Thus, a method for diagnosing high-grade cervical disease that is independent of or works in conjunction with conventional Pap smears and molecular testing for high-risk HPV infection is needed. Such a method should be able to specifically identify high-grade cervical disease that is present in all patient populations, including those cases classified as LSIL or CINI by Pap staining that are actually positive for high-grade lesions (i.e., “false negatives”). Therefore, there is a need in the art for specific, reliable diagnostic methods that are capable of detecting high-grade cervical disease and of differentiating high-grade disease from conditions that are not considered clinical disease, such as early-stage HPV infection and mild dysplasia.